[clang][analyzer] Display notes in StdLibraryFunctionsChecker only if interesting

The note tag that was previously added in all cases when a standard function call
is found is displayed now only if the function call (return value) is "interesting".
This results in less unneeded notes but some of the previously good notes disappear
too. This is because interestingness is not always set as it should be.

Reviewed By: donat.nagy

Differential Revision: https://reviews.llvm.org/D153776